Because of subtle chemical differences, age, and a variety of other factors, the teeth of patients in need of dental crowns vary in color to a significant degree. Porcelain is commonly used in the manufacture of dental crowns, and in order to properly match the color of a dental crown with the tooth color of a given patient, many porcelain formulations are currently available from which the dentist or orthodontist may choose. These porcelain formulations are supplied by their manufacturers in the form of white powders and are mixed with water to make a paste which is then molded to crown shape and fired. While color tabs are available to show the color of a formulation supplied under a given identification number, the actual color will usually vary slightly but noticeably from batch to batch. It is therefore the better practice to make a test color sample for each batch, and fire it; this insures a more precise determination of the color of a crown made using that porcelain batch. This procedure allows the dentist to choose or mix a porcelain formulation of the appropriate shade for a particular patient before the crown is actually made.
One prior art device for making such test samples or "tabs" is shown in FIG. 1, where it is labeled as prior art. That device consists of upper and lower rectangular plates which are pinned at one end to permit lateral sliding movement relative to one another. The upper plate includes a U-shaped cut out at its free end, the end being beveled so that its thickness diminishes toward the edge of the cut-out. In forming a color test sample or tab with that device, a porcelain formulation in the form of a moldable paste in packed in the cut out so that it generally conforms to the edgewise shape of the cut out. The paste is quite sticky, and tends to cling to the lower plate. It is dislodged without breaking by sliding the upper plate laterally across the lower plate until the plates are separated. The test tab (shown in FIG. 1a) can be carefully removed by pressing it transversely from the U-shaped cut out, and can then be fired.
A difficulty with that device is that the color of the porcelain test sample thereby formed does not match the color of an actual crown made from the same formulation, as closely as is desirable. Under inspection by a trained ceramist, a color difference is frequently apparent between a porcelain test sample as heretofore made, and a crown made from the same formulation.
An important aspect of this invention is the determination that a test sample or tab having curved and tapered surfaces approximating the configuration of a crown will provide a better indication of the true color of the crown formed from a given porcelain formulation, than will a flat tab. However, prior to this invention, it has proved difficult to make such a curved test sample or tab. The compacted, unfired porcelain tab formed in a curved mold is strongly adherent to the mold and has not easily been removed without crumbling.